1. Field of the Invention
The invention relates to a process for making multi-walled carbon nanotubes from a fly ash obtained from the combustion of a hydrocarbon mineral. The invention includes a low pressure chemical vapor deposition process in which an acetylene and carbon monoxide mixture is contacted with an electron beam treated crude oil fly ash. The invention further relates to a multi-walled carbon nanotube obtained from a process that utilizes ash obtained from the combustion of a hydrocarbon mineral.
2. Description of the Related Art
The description of the related art provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present invention.
Carbon exists in several forms or allotropes of particular chemical structure. Fullerenes are one form of carbon and encompass structures such as spherical structure also know as buckminster fullerenes and tubular structures also known as nanotubes. Graphene is also a sheet form carbon, however existing only in a two-dimensional form absent the three-dimensional structure that accompanies carbon sheets which are rolled or interconnected to thereby form a three-dimensional structure. Fullerenes generally have a structure formed from as two-dimensional grapheme sheet which, when rolled end-to-end, exists in the form of a tube or other three dimensional structure. The outer shell portion of a tubular structure having dimensions on a nanometer scale, known as a nanotube, encompasses an inner hollow portion in which other molecules or elements may be present.
Carbon nanotubes can further be described as having single-walled or multi-walled structures. Single-walled nanotubes are generally tightly wrapped layers of graphene forming tubes with a diameter of as little as one nanometer with an essentially unlimited length. Multi-walled nanotubes, on the other hand, contain multiple layers of roiled graphene sheets such that a first inner tube may be wrapped with a plurality of additional grapheme sheets to provide a layered structure wherein a plurality of tubes are nested in one another. Alternately a multi-walled nanotube may have a structure in which a graphene sheet is rolled upon itself such that the same grapheme sheet forms multiple layers of the nanotube.
Carbon nanotubes have physical and chemical properties that are of great interest in many fields of technology. Physical properties such as strength and stiffness are of special interest in applications such as fillers for plastic materials. Nanotubes may find use as coatings in special applications that are resistant to penetration, thermal stress, chemical stress and/or friction.
The tubular structure of a nanotube offers the possibility of forming nano-channels which may be used to purify chemicals and/or structure to provide specific electronic properties. A nanotube wire in which one or more molecules or atoms is permitted to move in two directions can be envisaged as offering unique electronic properties and applications such as microelectronics.
Although carbon nanotubes may exist naturally and may be formed during processes in which naturally occurring hydrocarbon materials are combusted or present in a highly reducing environment, it is generally thought that naturally occurring carbon nanotubes are of insufficient purity and regularity to have any significant commercial or technological use. Several methods have been developed for synthesizing carbon nanotubes on both small and large scale. Arc discharge was used early in the development phase of carbon allotropes. Using high voltage and high current electrical discharge in which at least one electrode of an arc is made from carbon forms carbon material in the form of nanotubes.
Laser ablation has also been to form carbon nanotubes. Elemental carbon is vaporized using a high energy laser in the presence of a cooling gas. As the vaporized carbon condenses in the cooling gas carbon nanotubes are formed and are collected by precipitation. By careful selection of conditions of temperature and laser energy, particular forms of carbon nanotubes can be isolated. Laser ablation offers substantially improved yields in comparison to arc discharge, however, laser ablation must be operated under conditions which are substantially more expensive and, in certain circumstances, less selective than the arc discharge method of making carbon nanotubes.
Chemical vapor deposition (CVD) has been used to make both single-walled and multi-walled carbon nanotubes. This method utilizes a hydrocarbon gas mixture that is contacted with a metal-containing catalyst. Under the correct conditions the hydrocarbon-containing gas decomposes and/or changes chemical formula when in contact with a heterogeneous catalyst such that carbon nanotubes are precipitated and/or formed on the catalyst particle. Conventional CVD techniques are amenable to forming carbon nanotubes of desirable structure and purity. Nonetheless CVD techniques have, so far, been difficult to scale to commercial quantities due to costs and throughput limitations.
The use of fly ash as a catalyst material for generating multi-walled carbon nanotubes has been described (see Dunens et al., Environ. Sci. Technol., 2009, 43 (20), pp. 7889-7894—incorporated herein by reference in its entirety). The use of particularly treated and/or chemically modified fly ash materials or fly ash materials obtained from the combustion of crude oil was not described. Likewise Yasui et al. (IEEJ Trans. Elec. Electron Eng., 2009, 4 (6) pp. 787-789) describe the use of a fly ash obtained from the combustion of coal. The authors did not describe the preparation of multi-walled carbon nanotubes from crude oil fly ash or the pre-treatment of a fly ash with electron beam bombardment. Irradiation effects on the leaching behavior and form of heavy metals in fly ash of municipal solid waste incinerator was described by Nam et al. (Journal of Hazardous Materials, Volumes 199-200, 15 Jan. 2012, pages 440-447).
The inventors have discovered that CVD processes can be used to form multi-walled carbon nanotubes in high yield and at low costs using carbon-containing fly ash obtained from the combustion of crude oil. The method described herein provides a low cost method for the industrial-scale production of multi-walled carbon nanotubes which utilizes materials as catalysts which would ordinarily be considered low value waste. The characteristics of multiwall carbon nanotubes have been reviewed by Lehman et al. (Carbon, Volume 49, Issue 8, Jul. 2011, pages 2581-2602).
In Saudi Arabia in particular the use of crude oil as a fuel source is widespread. The power plants in Saudi Arabia consumed 22 million metric tons of various types of fuel and fuel oil, especially heavy and thus large amounts of carbon-containing fly ash material is formed as a by-product from energy generation. Finding a use for the fly ash by-product provides many advantages that are limited only to getting rid of damage to the environment, but also for the use of the fly ash materials in the pipeline industry, e.g., for the formation of carbon nano-metric materials which have significant economic value.